Weathering, Soils, & Erosion

Earth Science Unit 7 (Chapter 14)

What is Soil

• Soil is a complex mixture of weathered rock, minerals, organic material (both living & decaying), water, and air.

• Different soils have different amounts.

• What might change the amounts?

• On average, soil has the following ratios:– 45% minerals & rocks

– 25% water

– 25% air

– 5% organic material

How is Soil Produced?

• Weathering of rocks and minerals

• Deposits of sediments washed/blown to the site

• Decomposition/actions of living things.

Weathering of Rock

• Weathering is the physical or chemical breakdown of rock into smaller pieces

• These pieces may be large or too small to see!

• Two types of weathering:

– Mechanical

– Chemical

Mechanical Weathering

• Physical breakdown of rock

• Chemical composition of the rock does not change

• Types:

– Exfoliation

– Ice Wedging

– Abrasion

– Plant & Animal Activity

Exfoliation

• When rocks come to the surface, pressure on them is reduced.

• This can allow cracks to form, and the rocks break apart.

• Example: Granite

Ice Wedging

• When water flows into the cracks in rocks & freezes.

• Why does this split the rocks?

Abrasion• When rocks broken from other processes

collide and break even more.

• Happens because of gravity, ice, running water, or wind…wait, how does wind do this?

Plant & Animal Activity• Plant roots act like ice to split rocks

• Animals, mostly ones that burrow, expose other rocks for further weathering.

Chemical Weathering

• Chemical reactions break the minerals in the rock into different materials

• Types:

– Oxidation

– Hydrolysis

– Carbonation

– Organic Acids

– Acid Rain

Oxidation• When elements

in the rock combine with Oxygen, and break off as new compounds.

• Usually happens in rocks with iron

• Produces a red color

Hydrolysis

• When chemicals in the rock react with water, causing some of the minerals to break away from the rock.

• Often happens with metals like Potassium & Aluminum

Carbonation• When CO2 gets into water it makes Carbonic Acid

• This acid can stick to some minerals and form compounds that are washed away by water.

• This often happens to limestone.

Organic Acids

• Some living things produce acids that are released to the environment.

• These acids can make cracks in rocks and start the weathering process.

• Examples: Lichens & mosses

Acid Rain

• Nitrogen & Phosphorus from fossil fuels get into the air and combine with rainwater

• This rainwater weathers rock more rapidly than normal rain.

Quick Quiz #1

• Why would rocks high in the mountains weather more quickly than rocks in a plain?

• You have 10 lbs. of rocks. 5 lbs. are in two rocks, and the other 5 lbs. are in 10 rocks. Which 5lb. Pile will weather more quickly?

• A stone statue in the desert will weather more ______________than a stone statue in PA.

• This happens when the softer areas of rock weather away before the harder areas or rock.

Formation of Soil

• When new rock is exposed, soil begins to form.

• This newly exposed rock is known as Bedrock.

Formation of Soil• Weathering begins to break the bedrock into

smaller rocks.

• This layer of partially weathered rocks, above the bedrock, is known as Regolith.

Formation of Soil

• The top of the Regolith is weathered even more, producing small particles that begin to form soil.

• Sediments from water or wind may add to the soil.

• Also, organic material begins to add to the soil.

Formation of Soil

Mature soil

Young soil

Regolith

Immature soil

Bedrock

Soil Characteristics

• The main characteristics of a soil are determined by the bedrock it formed from…this is the Parent Material.

• Parent material can affect soil texture and soil color.

• Example: quartz-rich rocks produce sandy soil.

• Example: iron-rich rocks produce red soil.

Soil Characteristics

• The size of soil particles also affects the characteristics of a soil.

• Soils are usually categorized by the amounts of each type of particle that they have.

• Particles are:

– Clay: less than 0.0002mm

– Silt: 0.0002mm – 0.05mm

– Sand: 0.05mm – 2mm

Effects of Particle Sizes

• The size of soil particles affects how much water & air the soil can hold.

• The smaller the particles, the less the soil can hold.

• However, if the soil is made entirely of larger particles, the water will run right through the soil!

• We measure particle sizes by measuring Porosity.

Porosity:

The measure of the volume of pores and distance of pores per volume of soil

Effects of Particle Sizes

• Infiltration: the rate at which water from precipitation or other sources flows into the soil

SaturationThe amount of water and air

that can move through soilWater Water

High permeability Low permeability

Soil Profiles

• The formation of soil in different areas produces different layers.

• These layers are called Horizons.

• The composition and depth of horizons is measured by looking at a Soil Profile.

Soil Horizons

O horizon

Leaf litter

A horizon

Topsoil

B horizon

Subsoil

C horizon

Parent

material

Mature soil

Young soil

Regolith

Immature soil

Soil Horizons

• O Horizon:

• Top Layer/Surface litter layer

• Mostly plant debris (leaves, branches) and animal waste

• Brown/black in color

Soil Horizons

• A Horizon

• The Topsoil Layer

• A mixture of decomposing organic matter called Humus

• And…

• Small soil particles from the bedrock

• Contains most plant roots and soil organisms.

Soil Horizons

• B Horizon (Subsoil Layer)

• A transitional layer of parent material, organic material, and minerals leeched from the A horizon.

• Leaching is when water reacts with minerals in the upper soil layers, carrying them to the B horizon.

Soil Horizons

• C Horizon (parent material):

• This is the regolith…which is…?

• The partially weathered rocks from the bedrock.

• This layer usually has no organic material.

• Below the C horizon is the bedrock.

Typical Pennsylvania Soil Profile

Other Soil Profiles

Different Soil Profiles in Rice Fields

What changes a Soil Profile?

• What determines the composition of a soil?

• Mostly the bedrock, erosion, & organic stuff…but what determines these?!?

• Climate!

• Climate mostly affects the layers of a soil, but not the materials found in the soil.

• In general, climates that have all 4 seasons have all the layers of soil.

Soil Layers & Climate

Tropical Rainforest Soils

• Large amounts of rain leech minerals from the upper layers to the B horizon, which becomes very thick.

• Almost no mechanical weathering…why?

• Decomposition is high…why?

• Would this be good for farming?

Tropical Soil Profile

Desert/Arctic Soils

• Very little rainfall, so almost no chemical weathering.

• Little life, so not much organic material.

• Soil is mostly regolith (C horizon) with a little organic material at the top (O horizon)

Desert Soil Profile

Temperate Soils

• This is where we live.

• Varying seasons provide all kinds of weathering.

• Lots of life, so there’s lots of organic material.

• Soils in these areas usually have all 4 layers.

Temperate Soil Profile

Soil Erosion

• What is erosion?

• The movement of weathered materials from one location to another.

• So…isn’t this the same as weathering?

• No! Weathering helps to build soil, soil erosion destroys soil.

What Causes Soil Erosion?

• Wind

• Gravity

• Water

• Glaciers

• Human Actions can speed up the process!

Wind Weathering

Wind Erosion

Gravity Weathering

Gravity Erosion

Water Weathering

Water Erosion

Glacier Weathering

Glacier Erosion

Humans & Erosion

• So, how do humans affect this Soil Erosion thing?

• Development for housing/buildings

• Farming…this is the big one

Humans & Erosion

• Farming can expose a lot of lose soil to erosion.

• This soil is the O and A horizons…why are these important?

• These contain most of the organic material in the soil, which plants and animals need to survive.

Farm Erosion

Is Erosion Really a Problem?

Areas of serious concern

Areas of some concern

Stable or nonvegetative areas

What can we do to prevent Erosion?

Contour Plowing

• Hillsides are plowed in curves that follow the shape of the land.

• Prevents water from flowing straight down.

Strip Cropping

• Crops are planted in alternating bands.

• This way, the whole area is not filled with row crops that don’t cover all the land.

Terracing

• Hills are carved into flat “steps”.

• This prevents water from flowing rapidly down the hill.

Crop Rotation

• Planting a crop to harvest 1 year, and one to leave in place the next year.

• Often combined with strip and contour farming.

Quick Quiz #2

• In your experiments, the variable you measured is known as the _______________ variable.

• This region would have a large B horizon in the soil, full of minerals leeched from the upper layers.

• These soil particles are the smallest.

• The measure of the rate of water movement into the soil.